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Catchment hydrological responses to forest harvest amount and spatial pattern - 2011
Citation:
Abdelnour, A., M. Stieglitz, F. Pan, AND R. B. MCKANE. Catchment hydrological responses to forest harvest amount and spatial pattern - 2011. WATER RESOURCES RESEARCH. American Geophysical Union, Washington, DC, 47:18 pages, (2011).
Impact/Purpose:
Policy makers, land managers and other decision makers are increasingly being asked to consider the effects of their decisions on multiple environmental endpoints, particularly in the context of how regulatory and management actions affect vital ecosystem services, i.e., the capacity of ecosystems to provide clean water, regulate greenhouse gases, produce food and fiber, provide habitat for fish and wildlife populations, amongst other services. Although computer simulation models are playing an increasingly important role in this regard, existing models have a number of disadvantages that limit their usefulness. Many are too simple to capture important process-level hydrological and biogeochemical controls on ecosystem responses to disturbance. At the other extreme, some models are so complex that they require forcing data that are often unavailable, or are too computationally expensive to extrapolate local dynamics over large areas, or require a high level of expertise to implement. Therefore, there is a need for a more balanced approach. Here we describe an ecohydrological model – VELMA (Visualizing Ecosystems for Land Management Assessments – that provides a relatively simple and efficient framework for assessing the effects of changes in land-use (harvest, fire, etc.), vegetation type, and climate on hydrological, ecological, and biogeochemical processes within watersheds. For this demonstration, we use VELMA to analyze how the amount and location of forest clearcutting within a watershed affects stream water quality. The model was previously calibrated to simulate observed ecohydrological responses of an intensively studied watershed in the H.J. Andrews Experimental Forest in Oregon. We apply 100 scenarios for which harvest amount ranged from 2% to 100% of watershed area. Model results show that (1) losses of ammonium and nitrate to the stream increased exponentially when unharvested riparian buffer zones fell below 60% of total catchment area, and (2) for each 1% increase in harvest area, dissolved organic carbon and nitrogen losses to the stream increased linearly. We also applied 20 scenarios for which harvest amount was fixed at 20% but harvest location varied with respect to hillslope position. As harvest distance to the stream decreased, simulated nitrate losses increased exponentially, and dissolved organic nutrient losses increased linearly. Our analysis examines how specific biogeochemical processes and hydrological processes interact within soil profiles and hillslopes to regulate short and long-term losses of nutrients following harvest. This exercise demonstrates VELMA’s potential for informing riparian forest management practices aimed at protecting stream water quality.
Description:
We used an ecohydrological model, Visualizing Ecosystems for Land Management Assessments (VELMA), to analyze the effects of forest harvest location and amount on ecosystem carbon (C) and nitrogen (N) dynamics in an intensively studied headwater catchment (WS10) in western Oregon, USA. Our goal is to elucidate how the interaction of hydrological and biogeochemical processes within harvested and unharvested areas regulates losses of dissolved C and N from the terrestrial system to the stream and atmosphere. The model was previously calibrated to simulate observed ecohydrological responses of WS10 to a whole-catchment clearcut in 1975. Here we apply 100 scenarios for which harvest amount ranged from 2% to 100% of catchment area. Model results show that (1) NH4 and NO3 losses increased exponentially when unharvested riparian buffer zones fell below 60% of total catchment area, and (2) for each 1% increase in harvest area DON and DOC losses increased linearly. We then apply 20 scenarios for which harvest amount was fixed at 20% but harvest location varied with respect to hillslope position. As harvest distance to the stream decreased, simulated NH4 and NO3 losses increased exponentially, and DON and DOC losses increased linearly. Our analysis examines how specific biogeochemical processes (decomposition, nitrification, denitrification and plant N uptake) and hydrological processes (evapotranspiration, and vertical and lateral flow) interact within soil profiles and hillslopes to regulate short and long-term losses of nutrients following harvest. This exercise demonstrates VELMA’s potential for informing riparian forest management practices aimed at protecting stream water quality.